Halliday & Matthiessen (2014: 665):
(ii) On the other hand, there are realignments in the realisational relationship between semantic units and grammatical ones. According to Table 10-1, a sequence is realised by a clause complex; the combination of a figure, a proposition/proposal and a message is realised by a clause; and we could add further realisational correspondences: for example, a participant is realised by a nominal group, a process by a verbal group and a circumstance by an adverbial group or a prepositional phrase. But once these couplings between the two strata of the content plane have been established, ‘cross-couplings’ become theoretically possible.
For example, while sequences are realised by clause complexes and figures by clauses, it is theoretically possible that, under certain conditions, sequences would be realised by clauses – that is, as if they were figures. This is the possibility of metaphorical realisation, which has been taken up in English and many other languages, creating a more complex relationship between semantics and lexicogrammar than the one shown in Table 10-1.
For example, instead of saying we and this common homeland are spiritually and physically united, so we were deeply pained as we saw ..., we can say that spiritual and physical oneness we all share with this common homeland explains the depth of the pain we all carried in our hearts as we saw .... But the two forms of realisation are not, of course, synonymous, so the effect is one of expanding the meaning potential of the language.